1. Field of the Invention
The present invention relates to bulk starter media for cheese making characterized by improved buffering and bacteriophage inhibition without affecting physical stability.
2. Description of the Prior Art
In the commerical production of cheese, large vats of milk are treated with a milk clotting agent such as rennin and inoculated with acid producing bacteria and optionally flavor producing organisms to provide the desired acid conditions for cheese making as well as the desired flavor, body, odor and texture characteristics of the resulting cheese.
The term "acid producing bacteria" refers to bacteria capable of fermenting lactose or other similar carbohydrates to produce, mainly, lactic acid, such as Streptococcus lactis, Streptococcus cremoris, Lactobacillus bulgaricus, and Streptococcus thermophilus.
The term "flavor-producing bacteria" refers to bacteria capable of fermenting citric acid or citrates at a favorable pH with the production of flavor producing compositions illustrated by diacetyl, acetylmethylcarbinol, and volatile material such as carbon dioxide, such as Leuconostoc citrovorum (Streptococcus citrovorus) and Leuconostoc dextranicum (Streptococcus paracitrovorus) and subspecies thereof. Streptococcus diacetylactis produces both lactic acid and flavor constituents.
The bacteria or culture is generally propagated from a mother culture in large enough quantities of milk (about 1% of the final volume) to produce a bulk starter, which can then be used for fermenting the final batch of milk to produce the end product, i.e., cheese.
A good starter must produce lactic acid in the cheese vat at a vigorous and steady rate. "Slow" starters produce cheese of inferior quality. Some of the most significant causes of starter slowness and failure is (1) bacteriophage, hereinafter "phage" (2) bacteria of low viability, and (3) low bacterial cell population.
It has been found that the composition of the starter medium exerts a considerable influence on the loss of cell viability due to phage attack. Media constituents such as milk, amino acids, vitamins and tryptophan, and particularly calcium ions, may be necessary for full bacteriophage activity. If the starter culture is grown in a calcium ion deficient medium, the culture is protected because a phage, if present, will die out (Davis ibid, at page 215).
Various bulk starter media have been formulated to promote bacterial growth and inhibit phage. The ingredients of seven such media are listed in Table I below:
TABLE I ______________________________________ General Composition of Several Bulk Starter Media.sup..circle.1 Ingredient present (+) or absent (-) Ingredient A B C D E F G ______________________________________ NFDM + + + + + + - Dextrose + + + - + - - Ammonium + + + - + - - phosphate Dextrin + + + - + - - Sodium + + + - + - - phosphate + + + Starch + - - - + - - Demineralized + - - - - + + whey powder Pancreas + + - - - - - extract Lactose - - + + - - - Whey - - + - + - - Powder Sucrose - - + - - - - Yeast - - + + + + + Extract Sodium - - - - + - - Citrate Hydrolyzed - - - - - + - cereal solids.sup..circle.2 Phosphate- - - - + - + + citrate buffer ______________________________________ .sup..circle.1 T. J. Gulstrom et al., J. Dairy Sci. 62:208-221 (1979) .sup..circle.2 Cereal not identified
Another bulk starter medium, disclosed in Canadian Pat. No. 1,024,393, can be formulated by blending 4.14 kilograms (9.2 lbs.) of the following mixture with 380 liters (100 gallons) of whey:
______________________________________ 36.4% Sodium dihydrogen phosphate 34.5% Disodium phosphate 19% Autolyzed yeast extract 9% Hydrolyzed casein 0.48% Magnesium sulfate 0.1% Ferrous sulfate 0.1% Manganese sulfate 0.1% Sodium chloride 99.78% ______________________________________
The autolyzed yeast extract and the hydrolyzed casein used in this medium acts as a stimulant for the growth of lactic acid producing bacteria. Dextrose and other sugars and dextrins are useful to accelerate the initial development of the culture and reduce the initial lag phase of the culture in fermenting lactose. Trace minerals such as magnesium (less than 1%) and manganese as well as ferrous sulfate are known in the art to be beneficial to lactic acid culture systems.
For phage control, the media examples above have been formulated without calcium or with calcium binding systems. It is well known that orthophosphate salts are effective in binding the calcium to prevent the calcium from promoting phage growth (Davis ibid, pp. 215, 237 and 238; Canadian Pat. No. 1,024,393; R. E. Hargrove et al, 1961, Journal of Dairy Science 44 pp. 1799-1810). Citrate buffers are also effective in phage control (Gulstrom et al., ibid; Anderson et al., U.S. Pat. No. 3,852,158 as well as Doull and Meanwell, Procedures, 13th International Dairy Congress, 3:114 (1953); and Rountree, Australian J. Experimental Biology and Medical Science 25, p. 203 (1947).
Even though some phage control can be effected with orthophosphates and citrates, the high concentrations of these salts which are required for phage control also suppress the growth of lactic acid producing bacteria as compared to their growth in skim milk and other media of relatively low ionic strength.
Salts that lower the ionic calcium to below 0.1 ppm. to achieve phage control often also cause physical instability of the starter, slow the starter activity or both and may weaken the gel strength of cheese curd.
A commercially useful bulk starter medium preferably should be formulated to preserve the stability of the protein in the medium upon heating. All bulk starter media must be pasteurized prior to inoculation. An improper selection of salts could cause the protein of a medium to form a curd or precipitate upon pasteurization. A heavy protein precipitate in a starter usually results in the undesirable discoloration of the curd in the bottom of the cheese vat.
Further, skim milk and most other starter media have limited acid buffering capacity from protein, salts or both. The speed of acid development when bulk starter is added to the cheese vat is a function of the total number of viable organisms in the starter as well as their position in the growth phase. Since bacterial growth is pH dependent, the number of bacteria which can be achieved in a starter and their position in the growth phase at the time of inoculation into the cheese vat is dependent upon the number of generations that can be produced before the pH of the medium drops to a value that slows, stops or inhibits the log phase of bacterial growth.
Three systems for controlling acid development and pH are: continuous neutralization by the external addition of base (gaseous ammonia or ammonium hydroxide) during culture growth, use of a highly buffered medium and neutralization after growth of the culture (pH Control During Lactic Starter Production, N. F. Olson, Dairy Field, May, 1981, pp. 92 and 94; see also Quality Sweetness Control and Marketing Strategy Highlight the ACDPI Clinic, Rita McNiece, Dairy Field, May 1981, pp. 86 et seq at pg. 87.) External neutralization is discussed in a paper entitled Lactic Bulk Culture System Utilizing a Whey-based Bacteriopage Inhibitory Medium and pH Control, G. H. Richardson et al., Journal of Dairy Science, Vol. 60, No. 3, pp. 378-386, which describe the injection of gaseous ammonia or liquid ammonium hydroxide to control pH.
Internally buffered bulk starter media have also been formulated. A presently available commercial internally buffered starter medium contains pretested sweet dairy whey, phosphate-citrate buffers and autolyzed yeast (from label declaration of ingredients). An analysis of the product shows the major constituents to be magnesium, phosphorus and sodium. The exact composition of this internally buffered starter medium is not known. However, this starter medium suffers from various defects. The constituents of this starter tend to settle out and form insoluble lumps in the bottom of the starter vat if agitation is not maintained throughout starter propagation. The citrates encourage gas production during starter propagation which can result in the creation of undesirable foam during starter propagation.
Previous media have generally been formulated with an excessive amount of fermentable carbohydrates. This excess carbohydrate allows the bacteria to produce sufficient acid to overcome any buffer capacity present in the medium and thus the pH drops to a level determental to bacterial growth and viability.
In addition, most other starter media must be used within 16-20 hours from the time of inoculation, or refrigerated to extend their usable life.
It has now been found that these problems can be overcome by the present invention.